The invention relates to the general field of magnetic disk systems with particular reference to GMR based read heads and the stability of pinned layers therein.
The principle governing the operation of magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). In particular, most magnetic materials exhibit anisotropic behavior in that they have a preferred direction along which they are most easily magnetized (known as the easy axis). The magneto-resistance effect manifests itself as an increase in resistivity when the material is magnetized in a direction perpendicular to the easy axis, said increase being reduced to zero when magnetization is along the easy axis. Thus, any magnetic field that changes the direction of magnetization in a magneto-resistive material can be detected as a change in resistance.
The magneto-resistance effect can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of a spin valve structure are shown in FIG. 1. In addition to a seed layer 12 on a substrate 11 and a topmost cap layer 17, the key elements are two magnetic layers 13 and 15, separated by a non-magnetic layer 14. The thickness of layer 14 is chosen so that layers 13 and 15 are sufficiently far apart for exchange effects to be negligible (the layers do not influence each other""s magnetic behavior at the atomic level) but are close enough to be within the mean free path of conduction electrons in the material.
If, now, layers 13 and 15 are magnetized in opposite directions and a current is passed though them along the direction of magnetization (such as direction 18 in the figure), half the electrons in each layer will be subject to increased scattering while half will be unaffected (to a first approximation). Furthermore, only the unaffected electrons will have mean free paths long enough for them to have a high probability of crossing over from 13 to 15 (or vice versa). However, once these electron xe2x80x98switch sidesxe2x80x99, they are immediately subject to increased scattering, thereby becoming unlikely to return to their original side, the overall result being a significant increase in the resistance of the entire structure.
In order to make use of the GMR effect, the direction of magnetization of one the layers 13 and 15 is permanently fixed, or pinned. In FIG. 1 it is layer 15 that is pinned. Pinning is achieved by first magnetizing the layer (by depositing and/or annealing it in the presence of a magnetic field) and then permanently maintaining the magnetization by over coating with a layer of antiferromagnetic material, or AFM, (layer 16 in the figure). Layer 13, by contrast, is a xe2x80x9cfree layerxe2x80x9d whose direction of magnetization can be readily changed by an external field (such as that associated with a bit at the surface of a magnetic disk).
The structure shown in FIG. 1 is referred to as a top spin valve because the pinned layer is at the top. It is also possible to form a xe2x80x98bottom spin valvexe2x80x99 structure where the pinned layer is deposited first (immediately after the seed and pinning layers). In that case the cap layer would, of course, be over the free layer.
As discussed above, the pinned layer (typically CoFe or similar ferromagnetic material) in the spin valve structures has to be exchange-biased by an AFM material. When pinned by MnPt or NiMn (AFM materials with high blocking temperature), the pinned layers usually display large anisotropy. The anisotropy field, Hck, is comparable to the pinning field Hpin, both these parameters being distributed over a range of values. These features result in pinned layer loop open and instability. This problem is more severe for the NiCr or NiFeCr seeded SVs in comparison to Ta seeded SVs.
It is also known that SVs made of a synthetic anti-parallel pinned layer (SyAP) can significantly reduce the loop open in the pinned layer. The pinning strength of a SyAP SV is much higher than that of the regular single SV. Typically, in the SyAP SV, AP1 and AP2 (two anti-parallel layers with AP2 being the layer close to the AFM) are coupled together through a layer of Ru and rotate coherently. This causes the Hck effect from AP2 to be greatly reduced. While this approach is a definite improvement on the state of the art, there are still loop opens in some cases (see later).
A routine search of the prior art was conducted. The following references of interest were found:
In U.S. Pat. No. 6,122,150, Gill shows a SV with two anti-parallel CoFe layers separated by a layer of Ru and capped with NIO. Iwasaki et al. in U.S. Pat. No. 5,738,946 show a SV MR with a NiCr protective film, while in U.S. Pat. No. 6,115,224, Saito discloses a SV MR process. Additionally, Iwasaki et al. (U.S. Pat. No. 5,702,832), Nakamoto et al. (U.S. Pat. No. 5,936,810), and Iwasaki et al. (U.S. Pat. No. 5,780,176) all show related MR processes and structures.
It has been an object of the present invention to provide a spin valve structure that has greater pinned layer robustness than is found in spin valves of the existing known art.
It has been an object of the present invention to provide a spin valve structure that exhibits a minimum amount of open looping in its hysteresis curve.
Another object of the invention has been to provide spin valve that is highly suitable for use in high density recording.
A further object of the invention has been to provide a process for the manufacture pf said spin valve and pinned layer.
These objects have been achieved by a using a modified pinned layer that is a laminate of five layersxe2x80x94a first layer of cobalt-iron, a layer of ruthenium, a second layer of cobalt-iron, a layer of nickel-chromium, and a third layer of cobalt-iron. The second layer of cobalt-iron should be about twice the thickness of the third cobalt-iron layer. The sum of the second and third cobalt-iron layer thicknesses may be greater or smaller than the thickness of the first cobalt-iron layer. A process for manufacturing the structure is also disclosed.